1. Field of the Disclosure
The present disclosure relates to a keyless entry system that includes a vehicle-side device and a portable device performing a wireless communication with each other, thereby locking or unlocking a door. In particular, the present disclosure relates to a keyless entry system that can determine with high precision whether a portable device is located inside or outside a vehicle.
2. Description of the Related Art
In the past, a keyless entry system is known including a vehicle-side device installed in a vehicle and a portable device carried by a user which perform a wireless communication with each other, thereby locking or unlocking a door of the vehicle. In recent years, a passive keyless entry system is known in which as a portable device approaches a vehicle, a communication is automatically established between a vehicle-side device and the portable device to perform authentication of ID set unique to the individual portable devices, thereby locking or unlocking a door of the vehicle. For example, such a keyless entry system is disclosed in U.S. Pat. No. 7,224,980 (corresponding to JP-A-2002-77972).
In the case of the passive keyless entry system, it is important to determine whether the portable device is located inside or outside the vehicle. For this reason, plural transmission antennas are installed at respective locations in the vehicle. Then, the portable device detects the intensity of the signals received from the respective transmission antennas and calculates the position of the portable device based on the detected intensity, thereby determining whether the portable device is inside or outside the vehicle.
The transmission antennas installed in the vehicle are provided at least at locations such as a door mirror on the outer body of the vehicle. The transmission antennas installed at locations on the outer body of the vehicle are connected to a circuit unit having a control unit via signal lines wired within the vehicle. Here, electromagnetic waves leak from the signal lines along which signals are transmitted. Therefore, when the portable device is located in the vicinity of the signal line connecting the circuit unit and the transmission antennas, the signal intensity detected by the portable device may become greater than its actual intensity since the intensity of signals leaking from the signal lines is added to the intensity of the signals transmitted from the transmission antennas. In such a case, when the distance to the transmission antenna is calculated based on the signal intensity detected by the portable device, the calculated distance may be smaller than the actual distance. Therefore, it becomes practically impossible to calculate the position of the portable device based on the distances to the plural transmission antennas and to determine the position of the portable device.
Specifically, when signals are transmitted from three transmission antennas, a combination of intensity data of the signals transmitted from the three transmission antennas corresponds to a particular point at which the portable device is located. However, when the reception intensity is increased greater than the actual intensity, there is a case where the distance to the portable device corresponding to the intensity of the signal transmitted from one transmission antenna does not overlap the position of the portable device determined by the intensities of the signals transmitted from the remaining two transmission antennas. In such a case, the combination of the intensity data does not correspond to a particular point. The same statement can be applied to the case where signals are transmission from two transmission antennas. The amount of leakage from the signal line differs from vehicle to vehicle and mainly depends on the way the signal lines are attached. Therefore, it is difficult to normalize the leakage amount.